A test method for determining the direct tensile strength of concrete based on a tension-compression bar model
By setting a step opening at the mid-span of the concrete beam specimen and configuring a tension/compression bar model, the problems of uneven stress distribution and error in concrete tensile strength testing were solved, achieving high-precision tensile strength determination and simplifying the operation process.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- CCCC FOURTH HARBOR ENG CO LTD
- Filing Date
- 2026-04-15
- Publication Date
- 2026-07-10
AI Technical Summary
Existing methods for testing the tensile strength of concrete suffer from uneven stress distribution, localized stress concentration, and the need for empirical conversion, which introduces errors and makes it difficult to balance testing accuracy with ease of operation.
A tension-compression bar model was used. By setting a step at the mid-span of the concrete beam specimen, configuring tension bars and compression bars, and applying pressure until failure, the direct tensile strength of the concrete was determined by combining parameters such as elastic modulus and moment of inertia.
This method enables the specimen to operate under approximately uniform tensile stress, and the test results are close to those of direct tensile strength. It is easy to operate and improves the accuracy and reliability of the test.
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Figure CN122361084A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the technical field of concrete testing, and in particular to a test method for determining the direct tensile strength of concrete based on a tension-compression bar model. Background Technology
[0002] Tensile cracking is one of the most common forms of damage to concrete structures during service, directly affecting their durability, serviceability, and load-bearing capacity. The tensile strength of concrete, as a key indicator for evaluating the development and control of cracks in a structure, is of significant reference value for performance assessment and maintenance decisions of in-service structures. Accurately determining the tensile strength of concrete helps to reasonably predict crack development patterns and avoid problems such as reduced structural load-bearing capacity, shortened service life, and increased maintenance costs caused by neglecting tensile properties.
[0003] Currently, there are three main methods for measuring the tensile strength of concrete: direct tensile test, splitting tensile test, and flexural test. The direct tensile test directly obtains tensile strength by axially stretching the specimen. However, due to the tendency for localized stress concentration to occur when the specimen is clamped at both ends, and the difficulty in ensuring perfect alignment during loading, the formation of secondary bending moments leads to significant dispersion in the test results, making it difficult to accurately reflect the true tensile properties of the material. The splitting tensile test is an indirect measurement method. It applies radial compressive stress to a cylindrical or cubic specimen, causing an approximately uniform tensile stress on the splitting surface. However, this method results in uneven stress distribution during loading, and the specimen is in a state of overall compression, leading to a measured tensile strength that is typically 10% to 15% higher than the true value. The bending test involves applying bending loads at three or four points to subject the bottom fibers of the specimen to the maximum tensile stress. However, the tensile stress is linearly distributed along the height of the cross section, with the maximum stress concentrated at the tension edge. The specimen does not work under pure tensile stress. The final result needs to be converted from bending strength to tensile strength using empirical formulas. There is a certain error in the conversion process, which often results in the tensile strength value being 50% to 100% higher than the actual value, making it difficult to accurately reflect the true tensile performance of concrete. Summary of the Invention
[0004] The problem this invention aims to solve is to provide a test method for determining the direct tensile strength of concrete based on a tension-compression bar model, addressing the aforementioned shortcomings of existing technologies. This method solves the problems of uneven stress distribution, local stress concentration, or the need for empirical calculations that introduce errors, making it difficult to balance test accuracy and ease of operation. It has the advantages of enabling the specimen to work under approximately uniform tensile stress, producing test results close to the direct tensile strength, and being easy to operate.
[0005] The above-mentioned objective of this invention is achieved through the following technical solutions:
[0006] A test method for determining the direct tensile strength of concrete based on a tension / compression bar model includes the following steps:
[0007] S1 Prepare a concrete beam specimen, wherein the concrete beam specimen is constructed with a stepped opening at the mid-span, and tie rods and compression rods are arranged sequentially from bottom to top in the stepped opening;
[0008] S2 applies a pressure to the stepped opening of the concrete beam specimen obtained in S1 until the concrete beam specimen is destroyed, and records the pressure load on the concrete beam specimen. ;
[0009] S3 is based on the pressure load obtained from S2. Determine the mid-span bending moment of the concrete beam specimen. And based on the elastic modulus of the concrete beam specimen. Neutral axis height and moment of inertia And a predetermined tension / compression bar model, thereby determining the direct tensile strength of concrete.
[0010] Furthermore, in S1, the tension / compression member model beam is a standard bending beam with dimensions of 150mm × 150mm × 500mm. The lower opening of the stepped opening is used to form a thin concrete slab representing the tension member, and the upper opening of the stepped opening is used to place a steel plate representing the compression member. The stepped opening at the mid-span is designed to alter the stress transfer mechanism, making the bottom of the mid-span resemble a thin slab.
[0011] Furthermore, in S1, the lower opening width of the step is 175mm. When a mold with an opening width of 175mm is used, the stress distribution difference between the upper and lower surfaces of the tie rod is smaller compared to the stress distribution difference between the other two opening widths.
[0012] Furthermore, in S1, the thickness of the concrete slab representing the tie rod is three times the maximum particle size of the coarse aggregate.
[0013] Furthermore, in S1, the thickness of the concrete slab is 30 mm. By adopting the minimum thickness of the concrete tie rod member, the maximum thickness of the tie rod is limited to 30 mm (3 times the maximum particle size of the coarse aggregate) to make the final test results more accurate.
[0014] Furthermore, in S1, the elastic modulus of the steel plate is 200 GPa, and its dimensions are 300 mm × 150 mm × 10 mm. Because the distribution of aggregate during the casting of the concrete beam specimen may lead to uneven stress distribution during the test, a high-strength steel plate is selected on the compression side to ensure uniform stress distribution. The preferred elastic modulus of the steel plate is 200 GPa, and its dimensions are 300 mm × 150 mm × 10 mm.
[0015] Furthermore, in S1, the bottom wall of the step opening and the step wall are smoothly connected.
[0016] Furthermore, in S1, the radius of the arc of the smooth transition connection is 30mm to avoid stress concentration.
[0017] Furthermore, in S2, a universal testing machine with a load-bearing capacity of 1000kN and a loading strength of 0.01mm / s is used to apply the pressure.
[0018] Furthermore, in S3, the predetermined tension / compression bar model satisfies,
[0019] ;
[0020] ;
[0021] ;
[0022] ;
[0023] ;
[0024] ;
[0025] ;
[0026] in, For the mid-span bending moment, The loading arm (i.e., the distance from the point of application of the loading pressure to the bottom support of the concrete beam specimen). For pressure load, For the width of the tie rod, The width of the concrete beam specimen. This is the equivalent width coefficient. The elastic modulus of the tie rod. The elastic modulus of concrete. For concrete compressive strength, The height of the neutral axis (i.e., the distance from the neutral axis to the edge of the pressure zone). For the thickness of the tie rod, The thickness of the concrete beam specimen. For the moment of inertia, The effective height of the interface of the concrete beam specimen (i.e., the distance from the resultant point of the tie rod to the edge of the compression zone). This refers to the direct tensile strength of concrete.
[0027] In summary, the beneficial technical effects of the present invention are as follows:
[0028] 1. Because this invention uses a sealing treatment on the circumferential surface of the concrete specimen (except for the two opposite surfaces to be heated) to form a one-dimensional water-sealed field, and covers it with a heat insulation layer to form a one-dimensional thermally sealed field, and then uses two opposite surfaces of electric heating radiation plates for symmetrical heating, the heat flow is transferred one-dimensionally along the thickness direction of the specimen, which effectively limits the generation of stress field orthogonal to the direction of thermal gradient, and avoids the interference of thermal stress on the concrete destructive behavior. Therefore, it can accurately measure the splitting tensile strength of concrete under the condition that it is only affected by pore pressure, and achieves effective decoupling of pore pressure and thermal stress.
[0029] 2. In this invention, it is preferable to adhere aluminum foil to epoxy resin to form a sealing layer. The incorporation of aluminum foil promotes the air curing of epoxy resin, avoiding the generation of bubbles due to steam flow on the surface of the test block during heating, thus ensuring the integrity and airtightness of the sealing layer. At the same time, it is preferable to cut the aluminum foil along the predetermined splitting surface and seal the cut with heat-resistant silicone before the splitting test to prevent the sealing layer from affecting the tensile strength test results of the test block, thereby further improving the accuracy and reliability of the splitting strength measurement.
[0030] 3. The method of the present invention first performs staged curing on the concrete test block (first sealed curing for 1 week, then open curing for 3 weeks, and finally sealed again for testing) to make the moisture content of the test block close to the normal service conditions and maintain moisture balance. Then, the central pore pressure of the test block is uniformly developed to the maximum value at a heating rate of 8-12℃ / min. The splitting test is immediately carried out in the temperature range of 175-225℃ where the pore pressure peak occurs. This method realizes the controllable and repeatable determination of the splitting tensile strength of concrete under different pore pressure levels, and provides a reliable test method for studying the high-temperature bursting mechanism of concrete. Attached Figure Description
[0031] Figure 1 This is a structural schematic diagram of the concrete beam specimen of Embodiment 1 of the present invention.
[0032] Figure 2 This is a schematic diagram showing the connection relationship between the concrete beam specimen, tie rod, and compression rod in Embodiment 1 of the present invention. Detailed Implementation
[0033] To make the technical means, creative features, objectives and effects of this invention clearer and easier to understand, the invention will be further described below in conjunction with the accompanying drawings and specific embodiments.
[0034] Example 1: This invention discloses a test method for determining the direct tensile strength of concrete based on a tension / compression bar model, comprising the following steps:
[0035] S1 reference Figure 1 Prepare a concrete beam specimen, which is constructed with a stepped opening at the mid-span, as shown in the reference. Figure 2 Within the stepped opening, tie rods and compression rods are arranged sequentially from bottom to top; wherein,
[0036] To change the stress transfer mechanism, the tension-compression bar model beam is a standard bending beam with dimensions of 150mm×150mm×500mm. Setting a step at the mid-span can make the bottom of the mid-span resemble a thin plate.
[0037] The lower opening of the step has a width of 175 mm and is used to form a thin concrete slab representing the tie rod. The minimum thickness of the concrete tie rod component is adopted, and the maximum thickness of the tie rod is limited to 30 mm (3 times the maximum particle size of the coarse aggregate) to make the final test results more accurate.
[0038] The upper opening of the step is larger than the lower opening and is used to place the steel plate representing the compression member. Since the distribution of aggregate during the casting of concrete beam specimens may lead to uneven stress distribution during the test, a high-strength steel plate is selected on the compression side to make the stress distribution uniform. The elastic modulus of the steel plate is preferably 200GPa, and the size is 300mm×150mm×10mm.
[0039] The bottom wall of the step and the step wall are smoothly connected with a radius of 30mm to avoid stress concentration.
[0040] S2 uses a universal testing machine with a bearing capacity of 1000kN and a loading strength of 0.01mm / s to apply a pressure to the stepped opening of the concrete beam specimen obtained in S1 until the concrete beam specimen is destroyed, and the pressure load of the concrete beam specimen is recorded. ;
[0041] S3 is based on the pressure load obtained from S2. Determine the mid-span bending moment of the concrete beam specimen. And based on the elastic modulus of the concrete beam specimen. Neutral axis height and moment of inertia And a predetermined tension / compression bar model, thereby determining the direct tensile strength of concrete; wherein, the predetermined tension / compression bar model satisfies,
[0042] ;
[0043] ;
[0044] ;
[0045] ;
[0046] ;
[0047] ;
[0048] ;
[0049] in, For the mid-span bending moment, The loading arm (i.e., the distance from the point of application of the loading pressure to the bottom support of the concrete beam specimen). For pressure load, For the width of the tie rod, The width of the concrete beam specimen. This is the equivalent width coefficient. The elastic modulus of the tie rod. The elastic modulus of concrete. For concrete compressive strength, The height of the neutral axis (i.e., the distance from the neutral axis to the edge of the pressure zone). For the thickness of the tie rod, The thickness of the concrete beam specimen. For the moment of inertia, The effective height of the interface of the concrete beam specimen (i.e., the distance from the resultant point of the tie rod to the edge of the compression zone). This refers to the direct tensile strength of concrete.
[0050] Example 2: This is a test method for determining the direct tensile strength of concrete based on a tension-compression bar model disclosed in this invention. The difference from Example 1 is that in S1,
[0051] The lower opening of the step has a width of 170 mm and is used to form a thin concrete slab representing the tie rod. The minimum thickness of the concrete tie rod member is adopted, and the maximum thickness of the tie rod is limited to 25 mm to make the final test results more accurate.
[0052] The upper opening of the step is larger than the lower opening and is used to place the steel plate representing the compression member. Since the distribution of aggregate during the casting of concrete beam specimens may lead to uneven stress distribution during the test, a high-strength steel plate is selected on the compression side to make the stress distribution uniform. The elastic modulus of the steel plate is preferably 180GPa, and the size is 300mm×150mm×10mm.
[0053] The bottom wall of the step and the step wall are smoothly connected with a rounded transition radius of 25mm to avoid stress concentration.
[0054] Example 3: This is a test method for determining the direct tensile strength of concrete based on a tension-compression bar model disclosed in this invention. The difference from Example 1 is that in S1,
[0055] The lower opening of the step has a width of 178 mm and is used to form a thin concrete slab representing the tie rod. The minimum thickness of the concrete tie rod component is adopted, and the maximum thickness of the tie rod is limited to 28 mm to make the final test results more accurate.
[0056] The upper opening of the step is larger than the lower opening and is used to place the steel plate representing the compression member. Since the distribution of aggregate during the casting of concrete beam specimens may lead to uneven stress distribution during the test, a high-strength steel plate is selected on the compression side to make the stress distribution uniform. The elastic modulus of the steel plate is preferably 210 GPa, and the size is 300mm×150mm×10mm.
[0057] The bottom wall of the step and the step wall are smoothly connected with a radius of 32mm to avoid stress concentration.
[0058] Example 4: This is a test method for determining the direct tensile strength of concrete based on a tension / compression bar model disclosed in this invention. The difference from Example 1 is that in S1,
[0059] The lower opening of the step has a width of 180 mm and is used to form a thin concrete slab representing the tie rod. The minimum thickness of the concrete tie rod member is adopted, and the maximum thickness of the tie rod is limited to 32 mm to make the final test results more accurate.
[0060] The upper opening of the step is larger than the lower opening and is used to place the steel plate representing the compression member. Since the distribution of aggregate during the casting of concrete beam specimens may lead to uneven stress distribution during the test, a high-strength steel plate is selected on the compression side to make the stress distribution uniform. The elastic modulus of the steel plate is preferably 220GPa, and the size is 300mm×150mm×10mm.
[0061] The bottom wall of the step and the step wall are smoothly connected with a radius of 35mm to avoid stress concentration.
[0062] Example 5: This invention discloses a test method for determining the direct tensile strength of concrete based on a tension-compression bar model. The difference from Example 1 is that the method of this invention is compared with three other conventional test methods. All methods use the test standard and loading regime based on GB50010-2010 to determine the tensile strength of concrete beam specimens. The concrete beam specimens are C30 concrete with a water-cement ratio of 0.65. Each cubic meter of concrete includes 293 kg of water, 450 kg of cement, 1000 kg of fine sand, 700 kg of coarse aggregate, and 2.25 kg of water-reducing agent. The test results are shown in Table 1.
[0063] Table 1
[0064]
[0065] Example 6: This invention discloses a test method for determining the direct tensile strength of concrete based on a tension-compression bar model. The difference from Example 1 is that the method of this invention is compared with three other conventional test methods. All methods use the test standard and loading regime based on GB50010-2010 to determine the tensile strength of concrete beam specimens. The concrete beam specimens are C60 concrete with a water-cement ratio of 0.33. Each cubic meter of concrete contains 149 kg of water, 450 kg of cement, 1000 kg of fine sand, 700 kg of coarse aggregate, and 9.9 kg of water-reducing agent. The test results are shown in Table 2.
[0066] Table 2
[0067]
[0068] As shown in Tables 1 and 2, the test results of bending and splitting tensile tests are higher than those of the direct tensile test. Furthermore, the differences between the test results in the splitting tensile test and the bending test are approximately 40% and 70%, respectively. This phenomenon may be due to the specimens not operating under pure tensile stress, and the fact that the final results need to be converted to tensile strength using empirical formulas. Since the empirical formulas are also derived from regression, errors may still exist during the conversion. The results obtained by the tension-compression bar method proposed in this invention are close to the results of the direct tensile test, and the variation between the test results of concrete with different compressive strengths is minimal. The difference between the direct tensile test results and the tension-compression bar method results is only about 9%, indicating that the direct tensile strength test method for concrete based on the tension-compression bar model provided in this invention has a good level of accuracy.
[0069] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.
Claims
1. A test method for determining the direct tensile strength of concrete based on a tension / compression bar model, characterized in that: Includes the following steps, S1 Prepare a concrete beam specimen, wherein the concrete beam specimen is constructed with a stepped opening at the mid-span, and tie rods and compression rods are arranged sequentially from bottom to top in the stepped opening; S2 applies pressure to the step opening of the concrete beam specimen obtained in S1 until the concrete beam specimen is destroyed, and records the pressure load on the concrete beam specimen. Based on the pressure load obtained in S2, S3 determines the mid-span bending moment of the concrete beam specimen, and based on the elastic modulus, neutral axis height, and moment of inertia of the concrete beam specimen, as well as the predetermined tension-compression bar model, further determines the direct tensile strength of the concrete.
2. The test method for determining the direct tensile strength of concrete based on a tension / compression bar model according to claim 1, characterized in that: In S1, the tension / compression member model beam is a standard bending beam with dimensions of 150mm × 150mm × 500mm. The lower opening of the stepped opening is used to form a thin concrete slab representing the tension member, and the upper opening of the stepped opening is used to place a steel plate representing the compression member. The stepped opening at the mid-span is designed to alter the stress transfer mechanism, making the bottom of the mid-span resemble a thin slab.
3. The test method for determining the direct tensile strength of concrete based on a tension / compression bar model according to claim 2, characterized in that: In S1, the width of the lower opening of the step is 170~180mm.
4. The test method for determining the direct tensile strength of concrete based on a tension / compression bar model according to claim 3, characterized in that: In S1, the thickness of the concrete slab representing the tie rod is 2.5 to 3.5 times the maximum particle size of the coarse aggregate.
5. The test method for determining the direct tensile strength of concrete based on a tension-compression bar model according to claim 4, characterized in that: In S1, the thickness of the concrete slab is 25~35mm.
6. The test method for determining the direct tensile strength of concrete based on a tension / compression bar model according to claim 2, characterized in that: In S1, the elastic modulus of the steel plate is 180~220GPa, and the dimensions are 300mm×150mm×10mm.
7. The test method for determining the direct tensile strength of concrete based on a tension-compression bar model according to claim 6, characterized in that: In S1, the bottom wall of the step opening and the step wall are smoothly connected.
8. The test method for determining the direct tensile strength of concrete based on a tension / compression bar model according to claim 7, characterized in that: In S1, the radius of the arc of the smooth transition connection is 25~35mm.
9. The test method for determining the direct tensile strength of concrete based on a tension-compression bar model according to claim 1, characterized in that: In S2, a universal testing machine with a load-bearing capacity of 1000kN and a loading strength of 0.01mm / s is used to apply the pressure.
10. The test method for determining the direct tensile strength of concrete based on a tension-compression bar model according to claim 1, characterized in that: In S3, the predetermined tension / compression bar model satisfies, ； ； ； ； ； ； ； in, For the mid-span bending moment, The loading arm (i.e., the distance from the point of application of the loading pressure to the bottom support of the concrete beam specimen). For pressure load, For the width of the tie rod, The width of the concrete beam specimen. This is the equivalent width coefficient. The elastic modulus of the tie rod. The elastic modulus of concrete. For concrete compressive strength, The height of the neutral axis (i.e., the distance from the neutral axis to the edge of the pressure zone). For the thickness of the tie rod, The thickness of the concrete beam specimen. For the moment of inertia, The effective height of the interface of the concrete beam specimen. This refers to the direct tensile strength of concrete.